1. Field of the Invention
The present invention relates to a configuration of a semiconductor element including a field-effect transistor type and an electric circuit using the semiconductor element. Further, the present invention relates to a light emitting device wherein a light emitting element and the semiconductor element controlling the light emitting element are provided on the light emitting device. Or, the present invention relates to a display device. Moreover, the present invention relates to an electronic apparatus on which the light emitting device and the display device are mounted.
2. Description of the Related Art
In recent years, the importance of a light emitting device displaying images is increased. As the display device, a liquid crystal display device displaying images by using a liquid crystal element is widely used as a display device for various kinds of uses including a cellular phone, a personal computer and the like by utilizing the advantages of the liquid crystal display device such as high picture quality, thinness and lightweight.
On the other hands, the development of a light emitting device and a light emitting display device using a light emitting element which serves as a self-luminous element has also been advanced. Such self-luminous element includes various kinds of elements in the widely range of an organic material, an inorganic material, a thin film material, a bulk material, and a dispersion material.
Especially, a representative self-luminous element is an organic light emitting diode (OLED) element. An OLED display device using the OLED element as a light emitting element has, in addition to features that the OLED display device is thinner and lighter than the conventional liquid crystal display devices, new features such as a first response speed, a wide viewing angle and a low voltage drive which are suitable for an animation display. Therefore, the OLED display device is drawing attention as the next-generation display device since the OLED display device is considered wide range of uses such as a cellular phone, a portable information terminal (PDA: personal digital assistant), television, and a monitor.
In particular, an active matrix (AM) type OLED display device can provide a high definition display in a large size screen, which is difficult to a passive matrix (PM) type OLED display device. Furthermore, the AM type OLED display device operates at lower power consumption than the PM type OLED display device, and has a high reliability. Thus, the AM type OLED display device is expected to be implemented.
One of various conditions necessary for putting a light-emitting device such as an OLED display device into practice is to maintain the luminous intensity almost constant. Especially an OLED device has a problem that the luminous intensity greatly depends on ambient temperature. In many OLED devices, the amount of the current increases in high temperature under the condition that the voltage is constant. The greater the amount of the current flowing to the OLED device is, the higher the intensity of the OLED device becomes.
Then, the OLED light-emitting device would be unstable and very bad for usage such that the display of the whole screen changes in brightness in accordance with the change of temperature.
There is also a problem that an existing OLED device generally tends to be decreased, as the time elapses, in the luminous intensity due to light emission. This is a fairly serious problem although the degree of decrease of the luminous intensity is broad depending on the structure of the OLED device.
When the luminous intensity decreases as the time elapses due to the amount of light emission and thereby the luminous intensity cannot be maintained almost constant, the display of a light-emitting device is not only unstable in the brightness as a whole but also has a problem in displaying gradation in each pixel. For example, displaying in respective pixels in a screen of a static image having significant difference in the luminous intensity over a long time causes burning of an image, which becomes so ugly in appearance.
Especially in the case of an OLED display device for displaying a color image by means of three kinds of luminous element corresponding to R (red), G (green) and B (blue), used generally “a triple color painting method” in which an OLED device to be used is different by a color, from the view of high efficiency and low consumption current in emitting a light. Then, color shift occurs since the temperature dependency of the luminous intensity is different by a color. Otherwise, color shift between the displaying colors in a light-emitting device occurs because the luminous intensity of respective OLED devices decrease at a different speed corresponding to every color as the time elapse.
The decrease of the luminous intensity of an OLED device in elapse of time becomes larger in the case that the voltage applied to the OLED device is constant (constant voltage driving) than the case that the current flowing to the OLED device is constant (constant current-driving). The reason is as follows.
It is said that the luminous intensity L of an OLED device is generally proportional to the amount of the current flowing to the OLED device I (V). When the proportional constant is c (V), there is a relation expressed by L=c(V)I(V), wherein V is the applied voltage to the OLED device necessary for emitting a light at the intensity L.
Continuous emission of a light by an OLED device, however, gradually decreases both of c (V) and I (V). Here, in the case of constant voltage-driving of a OLED device, the decrease of the both of c (V) and I (V) is reflected to that of L. On the other hand, in the case of constant current-driving of an OLED device, the decrease of c (V) is only reflected to that of L. Therefore, the decrease of L is larger in comparison in the constant voltage-driving than in the constant current-driving.
As a background of the decrease of c (V), an OLED device originally has little resistance against moisture, oxygen, light and heat, and thereby, change of characteristics and deterioration of the device per se tend to start or be facilitated easily. However, the progressing speed of the deterioration of the device greatly depends on the kind of a luminous material, the material of an electrode, the structure of a device driving the light-emitting device, the manufacturing circumstance and the manufacturing conditions. Improving the above, therefore, can restrain to a certain degree the decrease of c (V) in elapse of time.
Further, the temperature dependency of the luminous intensity of an OLED device is significantly high in the constant voltage-driving whereas that is often low in the constant current-driving. This can be considered that I (V) has high temperature dependency while c (V) has little temperature dependency under a condition of 1=c(V)I(V).
In view of the above, gradation display by current-driving rather than the voltage-driving of a luminous element of an OLED light-emitting device must enable the luminous element to maintain the almost constant intensity without greatly decreasing the luminous intensity in the elapse of time and without depending on the change of ambient temperature.
In the case of luminous elements other than the OLED device, the temperature dependency is also low in the constant current-driving rather than in the constant voltage-driving, generally, although it also depends on the kind of the device. From this point of view, the constant current-driving is still more preferable.
In a light-emitting device such as an AM-type of OLED display device, the current-driving of a luminous element is possible by mounting a current storing circuit on a pixel. The current storing circuit to be mounted on a pixel can be produced by means of an active element such as a thin film transistor (TFT).
Not only the current storing circuit but a circuit in a pixel is generally preferable to have a structure as simple as possible from a view of reduction of a manufacturing cost and lowering of the defective rate.
Moreover, the area occupied by a circuit is preferably as small as possible since improvement of the luminous area rate (the aperture rate) is strongly required in order to save power and stabilize emission of a light. The small luminous area rate, however, requires the luminous element to emit a light at the high current density for the purpose of obtaining the predetermined intensity, so that the luminous element would be easily facilitated to be changed in characteristics and deteriorate.
The most direct and effective method for improving the luminous area rate (the aperture rate) is to mount a circuit in a pixel on the side opposite to the direction of light-emission. This method, however, is not an effective solution in practice since it is better to mount a circuit in a pixel on the same side as the direction of light-emission in order to stably produce an OLED device.
Another reason why the small circuit area is preferable is that it is possible to highly integrate a pixel circuit for high functionalization.